<body>
        <p>如果时间早于 20:00，会获得问候 "Good day"。</p>   
        <button onclick="myFunction()">点击这里</button>
        <p id="demo"></p> 
        <script>
            function myFunction()
            {
                var x="";
                var time=new Date().getHours();
                if (time<20)
                {
                    x="GOOD DAY"
                }
                document.getElementById("demo").innerHTML=x;
            }
        </script>
        </body>
        </html>
        